1. Field Of The Invention
The invention relates to an ebullated bed process for the liquid phase hydrotreating of a hydrocarbon feedstock. The invention also relates to a recycle system with a liquid/liquid eductor for ebullating a catalyst bed.
2. Description Of Other Related Methods In The Field
The ebullated bed process comprises the passing of concurrently flowing streams of liquids or slurries of liquids and solids and gas upwardly through a vertically elongated cylindrical vessel containing a catalyst bed. The catalyst in the bed is maintained in random motion in the liquid and has a gross volume dispersed through the liquid greater than the volume of the catalyst when stationary. This technology has been used commercially in the upgrading of heavy liquid hydrocarbons or converting coal to synthetic oils.
The process is generally described in U.S. Pat. No. Re 25,770 to Johanson incorporated herein by reference. A mixture of hydrocarbon liquid and hydrogen is passed upwardly through a bed of catalyst particles at a rate such that the particles are forced into random motion as the liquid and gas flow upwardly through the bed. The random catalyst motion is controlled by recycle liquid flow so that at steady state, the bulk of the catalyst does not rise above a definable level in the reactor. Vapors along with the liquid which is being hydrogenated are removed at the upper portion of the reactor.
In an ebullated bed process the substantial amounts of hydrogen gas and light hydrocarbon vapors rise through the reaction zone into the catalyst free zone. Liquid from the catalyst free zone is both recycled to the bottom of the reactor to ebullate the catalyst bed and removed from the reactor as product. Vapor is separated from the liquid recycle stream before being passed through the recycle conduit to the recycle pump suction. The recycle pump (ebullation pump) maintains the expansion (ebullation) and random motion of catalyst particles at a constant and stable level by recycling liquid to the bottom of the reactor. The recycle pump for this service has a high suction pressure but a head of only 10 to 100 psi, typically 50 psi. Such pumps are commercially available and command a premium price. Because of the high suction pressure service, the support facilities for these pumps, e.g., high pressure seal oil system and variable speed drive system require frequent and expensive maintenance.
Reactors employed in a catalytic hydrogenation process with an ebullated bed of catalyst particles are designed with a central vertical recycle conduit which serves as the downcomer for recycling liquid from the catalyst free zone above the ebullated catalyst bed to the suction of a recycle pump to recirculate the liquid through the catalytic reaction zone. The recycling of liquid from the upper portion of the reactor serves to ebullate the catalyst bed, maintain temperature uniformity through the reactor and stabilize the catalyst bed.
U.S. Pat. No. 4,684,456 to R. P. Van Driesen et. al. teaches the control of catalyst bed expansion in an expanded bed reactor. In the process, the expansion of the bed is controlled by changing the reactor recycle pump speed. The bed is provided with high and low level bed detectors and an additional detector for determining abnormally high bed (interface) level. The interface level is detected by means of a density detector comprising a radiation source at an interior point within the reactor and a detection source in the reactor wall. Raising or lowering the bed level changes the density between the radiation source and the radiation detector. The vertical range of steady-state bed (interface) level as well as the highest and lowest steady-state interface level are design parameters.
U.S. Pat. No. 3,363,992 to M. C. Chervenak teaches a pumpless recycle system which is particularly adaptable to ebullating a catalyst bed. The recycle system relies on an elevated separator vessel to provide a liquid head in the absence of a recycle pump.
U.S. 3,617,524 to A. L. Conn teaches an ebullated bed process in which an eductor is used to inject a high velocity hydrogen stream into a hydrocarbon oil feedstock.